This disclosure generally relates to systems and methods for identifying a moving target at a remote location. In particular, this disclosure relates to measuring vibrations of a moving target for the purpose of target identification.
To track a moving target, radar devices typically detect the motion of the target based upon Doppler information provided by the radar signals that are reflected off the moving target. The movement of the target in a radial direction, relative to the radar device, causes the radar signals that reflect off the moving target to return to the radar device with a frequency that is different than the frequency that was transmitted by the radar device. Specifically, the radial movement of the target changes the frequency of the radar signal an amount that is proportional to the relative velocity of the target such that the change in frequency of the radar signal may be used to determine the location and speed of the moving target and to accordingly track the moving target.
A laser Doppler vibrometer (LDV) can be used to make non-contact vibration measurements of a surface. The laser beam from the LDV is directed at the surface of interest, and the vibration amplitude and frequency are extracted from the laser light reflected from the surface by detecting the Doppler shift due to the motion of that surface. The output of an LDV is generally a continuous analog voltage that is directly proportional to the target velocity component along the direction of the laser beam.
A typical vibrometer comprises a two-beam laser interferometer that measures the frequency (or phase) difference between an internal reference beam and a test beam. The test beam is directed at the target, and scattered light from the target is collected and interfered with the reference beam on a photodetector, typically a photodiode. Most commercial vibrometers work in a heterodyne regime by adding a known frequency shift (typically 30-40 MHz) to one of the beams. This frequency shift is usually generated by a Bragg cell, or an acousto-optic modulator. In vibrometers of this type, the beam from the laser, which has a frequency fo, is divided into a reference beam and a test beam using a beamsplitter. The test beam then passes through the Bragg cell, which adds a frequency shift fb. This frequency shifted beam is then directed toward the target. The motion of the target adds a Doppler shift to the beam given by fd=2*v(t)*cos(α)/A, where v(t) is the velocity of the target as a function of time, a is the angle between the laser beam and the velocity vector, and λ is the wavelength of the light. Light scatters from the target in all directions, but some portion of the light is collected by the LDV and reflected by the beamsplitter to the photodetector. This light has a frequency equal to fo+fb+fd. This scattered light is combined with the reference beam at the photodetector. The initial frequency of the laser is very high (>1014 Hz), which is higher than the response of the detector. The detector does respond, however, to the beat frequency between the two beams, which is at fb+fd (typically in the tens of MHz range). The output of the photodetector is a standard frequency-modulated signal, with the Bragg cell frequency as the carrier frequency, and the Doppler shift as the modulation frequency. This signal can be demodulated to derive the velocity versus time of the vibrating target.
The LDV described in the previous paragraph has at least the following limitations:
(1) The acousto-optic modulator Bragg cell has a maximum modulation frequency of ˜400 MHz, whereas a Mach 10 target has a Doppler shift of 4.4 GHz.
(2) The detection range is limited by the laser coherence length. Coherent heterodyne detection requires stand-off distances at less than half the laser coherence length.
(3) Mixing efficiency is low due to the complex optical train, namely beam splitters, which allow only part of the return signal to participate in the intermediate frequency mixing.
It would be desirable to provide an LDV for target identification which does not have the foregoing limitations.